Kirk also makes a base with a 3/8" screw thread that you can fit to any tripod head. The base in combination with their Mamiya 7 L bracket would allow you to use your Gitzo head. I carry one of these bases to convert my non-Arca tripod heads as needed.
RRS, Kirk, Acratech and Markins all use the Arca-Swiss type QR system. They all make high quality products and any of them can be fit to both 3/8" and 1/4" tripod screws.
I prefer the combination of quality, variety and service that RRS offers.
Thanks to all for your replies to my question. Someone in a private pm offered me a novoflex mc-vertikal at a good price and I am going to give it a try. If it works it will be a lot less expensive than any of the other options mentioned, especially since they would involve a new ball head when I am very happy with the one I have.